1. Field of the Invention
The present invention relates to a quantization control system for video coding. Particularly, the present invention relates to a quantization control system for video coding, for calculating a visual priority parameter which expresses human visual characteristics for videos with high precision and calculating a quantization parameter based on the visual priority parameter, thereby improving subjective image quality over the whole image even in a limited-bit-rate image distribution service.
2. Description of the Related Art
In a coding verification model Test Model 5 [1] of MPEG-2, which is an international standard for video coding, as shown in FIG. 8, there is provided a virtual buffer inputting the number of generated bits for each macroblock (identified as k) and outputting the number of target coded bits, and excess or deficiency of the generated bits relative to the number of target coded bits is stored in the virtual buffer to be fed back for quantization control.
Here, a weight factor for each macroblock (16×16 pixels), that is, a visual sensitivity w(k) is calculated by the following equation (1), and a quantization parameter is calculated by the following equation (2) using the visual sensitivity w(k) and an occupancy d of the virtual buffer. When the number of target bits of a picture and the number of macroblocks in a picture are indicated by T and MBcnt, respectively, the number of target coded bits is indicated by T/MNcnt.
                              w          ⁡                      (            k            )                          =                                            2              ×                                                          ⁢                              act                ⁡                                  (                  k                  )                                                      +            avg_act                                              act              ⁡                              (                k                )                                      +                          2              ×              avg_act                                                          (        1        )            where it is given act(k)=min {ax(k)}, ax(k): luminance distribution of a block, and avg_act: an averaged act(k) of a picture.Quantization parameter=Occupancy d×Visual sensitivity w(k)  (2)
However, in the above-mentioned conventional technique, the visual sensitivity is determined based on only luminance distribution of macroblocks, as seen from the equation (1), which presents a problem that weighting on each macroblock does not always match human visual sensitivity to videos.
Further, since the smallest luminance distribution ack(k) among luminance distributions ax(k) of blocks each of 8×8 pixels which constitutes a macroblock is used to calculate a visual sensitivity w(k) of the macroblock, when there exist a single flat block in a fine region, the calculated visual sensitivity is significantly different between a macroblock in which the flat block is included and a macroblock which includes no flat block, which presents a problem that the visual sensitivity varies widely between fine regions of almost similar quality.
Furthermore, for the case where quantization control is performed using a thus calculated quantization parameter, even though little noise is seen in the macroblock of the single flat block, the macroblock is subjected to fine quantization and accordingly, a large number of bits are allocated to the macroblock, presenting a problem that optimum bit allocation is not carried out.